Presently, the lasers that have been put into practical use are known to include gas lasers, such as the He—Ne laser and the Ar laser, solid state lasers, such as the Nd—YAG laser, dye lasers, and semiconductor lasers. FIG. 1 shows a relationship between wavelength band and output power of lasers. In recent years, compact, lightweight, and inexpensive semiconductor lasers have become popular in wavelength band 102 in the visible and infrared regions. Especially, in the optical communication filed, 1.3-μm band and 1.5-μm band semiconductor lasers for signal light sources and the 0.98-μm band and 1.48-μm band semiconductor lasers for fiber amplifier pumping have come into widespread use. Moreover, the semiconductor laser is used also as lasers for CD and red lasers, and the semiconductor laser is used also in wavelength band 101 in the visible and infrared regions used for reading and writing storage media, such as DVD and Blue-ray.
However, the semiconductor laser has not been put into practical use in wavelength band 111 of the green, yellowish green, and yellow ranges of wavelengths of 0.5-0.6 μm and in wavelength band 112 of the mid-infrared range 2-5 μm, and hence the gas laser and the solid-state laser, which are expensive and consume large electric power, are being used.
Optical characteristics, such as refractive index and absorption, of optical media of liquids, glasses, etc. have become important evaluation items to specify characteristics of optical instruments and to control qualities, such as accuracy and purity, of foods, medicines, etc. For measurement of these optical characteristics, the light source for generating the sodium D-line of wavelengths of 589-590 nm in the yellow range included in wavelength band 111 is being used.
For example, a relationship between the refractive index and the sugar content in a liquid is defined as Brix value by ICUMSA (International Commission for Uniform Methods of Sugars Analysis) and a method for finding the sugar content from measurement of the refractive index is provided. This method is applied to sugar content measurement of fruits and alcoholic beverage, being used widely industrially.
In the field of medicines, the Japanese pharmacopoeia defines refractive indices of solutions in which respective medical agents are solved as one of quality control measures of medical agents. There is a case where a “right-hand-system” medicine that has a spiral structure, such as thalidomide, may have a medicinal effect, but a “left-hand-system” medicine may serve as a poison. It is impossible to separate physicochemically substances each having mutually inverse spiral structure like this from each other. However, it is known that these substances exhibit different optical activities, and can easily be identified optically. Then, after phytotoxicity accidents like thalidomide, the Japanese pharmacopoeia defines a measurement of angle of rotation using the sodium D line. Medicines exhibiting such a property include a large number of medicines, such as menthol, prostaglandin, β lactam antibiotics, and quinolone antibacterial agents, besides thalidomide.
Presently, a laser light source for generating the sodium D line has not been realized, and a sodium vapor lamp or yellow LED is used as a light source. A light beam from a sodium vapor lamp is excellent in monochromaticity, but is a divergent light beam emitted in all directions. Therefore, it is difficult to collimate it, and so accurate measurement of optical characteristic is difficult. Moreover, since focused energy does not reach a high level, it is necessary to use a high-power lamp.
On the other hand, the spectral linewidth of the yellow LED is as wide as approximately 20 nm. Because of this, the spectral linewidth is intended to be narrowed by extracting a spectrum near the sodium D line using an optical filter, but there is a limit to narrow it. Moreover, since the yellow LED light lacks coherency, there is a limit in improving measurement accuracy.
In the context of such facts, improvement in accuracy of optical evaluation methods that have been prescribed with the sodium D-line wavelengths are being demanded in many industrial fields, such as quality control of foods and medicines. If a laser at the sodium D line can be realized, measurement using light interference will become possible. With the use of optical interference, measurement accuracy of the refractive index of various liquids and optical media including foods and medicines can be improved from the present value by about two orders of magnitude, and low consumption power and miniaturization become possible as well.
The electronic structure of sodium and characteristics of light generated from its energy transition will be described (see Non-patent document 1). It is known that wavelengths of emission from a sodium atom are 589.592 nm (D1 line) and 588.995 nm (D2 line). The D1 line and D2 line are collectively called D line, and the wavelength of D line may be called 589.3 nm, taking an average of the two wavelengths. FIG. 2 shows the energy levels of a sodium atom. The D line is generated accompanying a transition from the 3P level, which is the first excitation state, to the 3S level, which is the ground state. The 3P level has a fine structure of 3P1/2 and 3P3/2. Emission of the D1 line is caused by a transition from 3P1/2 to 3S1/2 and emission of the D2 line is caused by a transition from 3P3/2 to 3S1/2.
The 3S1/2, 3P1/2, and 3P3/2 levels have hyperfine structures due to interaction of the electron magnetic moment and the intrinsic magnetic moment of the atomic nucleus. The 3S1/2 level splits into two levels whose energy difference is 7.3 μeV, the 3P1/2 level splits into two levels whose energy difference is 0.78 μeV, and the 3P3/2 level splits into four levels whose energy difference is 0.48 μeV (maximum difference).
In order to realize a laser emitting light at the D1 line wavelength and the D2 line wavelength, it is necessary to create population inversions between energy levels corresponding to each light. In order to create a population inversion, it is necessary to construct a three-level system or four-level system. However, in the energy levels shown in FIG. 2, relaxation of 3P3/2 to 3P1/2 is a forbidden transition, and a relaxation time of 3P1/2 to 3S1/2 is 15.9 ns (Non-patent document 2). For example, when comparing it with a relaxation time of 3.2 μs in the TiAl2O3 laser, the former is shorter than the latter by two orders of magnitude or more. Therefore, it is difficult to create a population inversion between 3S1/2 and 3P1/2, so laser oscillation of the sodium D-line wavelength has not yet been realized. Alternately, although laser oscillation using the hyperfine structure is conceivable, the energy differences of the hyperfine structures of the 3S1/2, 3P1/2, and 3P3/2 levels in a sodium atom are about four orders of magnitude smaller than energy at room temperature (300K), 25.8 meV. Because of this, excitation at room temperature is distributed to the both split levels in the hyperfine structure almost equally, and cannot create a population inversion. For these reasons, lasers at the sodium D1 line and D2 line have not been realized until now.
Conventionally, semiconductor lasers have been put to practical use only in the wavelength bands of shorter than 500 nm and longer than 620 nm. In the wavelength band of 500-620 nm, solid-state lasers have been realized by a second overtone generation method using fiber lasers or the Nd—YAG laser, but a solid-state laser of an arbitrary wavelength has not yet been realized.
On the other hand, the second overtone generation method (SHG method) using a nonlinear crystal is known as a method for generating coherent light in the visible range. In order to generate light of the D1 line or D2 line by this method, light of the 1179.2-nm wavelength or the 1178.0-nm wavelength is required. Unfortunately, although these wavelengths can be attained by semiconductor lasers, it is extremely difficult to obtain a laser capable of delivering necessary power.
Visible light can also be obtained by generating sum frequency of two excitation laser beams with a nonlinear crystal. In this method, energy of sum frequency light is given by a sum of energies of the two excitation beams. This method comes with an advantage that freedom of a combination of wavelengths of the two excitation beams is widened because a desired wavelength is obtained by sum frequency generation. Therefore, it is the most practical method to realize a laser of an arbitrary wavelength. However, generally nonlinear optical phenomena had a problem of low efficiency. In order to solve this problem, selection of an existing laser device that can deliver high excitation light intensity and that is compact and consumes low electric power as well as improvement in characteristics of a nonlinear optical crystal become important.
The first object of this invention is to provide a laser light source that generates a coherent beam that is energy-efficient with a narrow linewidth and excellent directivity and generates a coherent beam of a wavelength of the sodium D line.
Conventionally, the laser microscope that scans a sample with a confocal laser beam to obtain an optical tomogram is known. The laser microscope is being used for analyzing distributions of a substance with fluorescent labeling in a tissue and cell. Moreover, there is known a flow cytometer that irradiates a laser beam onto a stream of cells aligned in a line, and analyzes and isolates a cell preparatively depending on fluorescence intensity. The flow cytometer is a measuring apparatus that uses a flow cytometry method for identifying a cell qualitatively using properties of a cell, for example, a size, a DNA content, etc. as optical parameters.
Although the fluorochrome is used as fluorescent labeling in recent years, since the fluorochrome was a foreign matter for cells, there are problems that the properties of a cell is affected, a cell dies, etc. Therefore, the method for performing fluorescent labeling with a green fluorescent protein extracted from jellyfish etc. is being used. Moreover, fluorescent proteins that exhibit florescence of yellow and red have been obtained by mutation and genetic manipulation of green fluorescent proteins (for example, see Non-patent document 3), and more detailed measurement and analysis are being conducted using multicolor fluorescence.
Since the red fluorescent protein has the absorption maximum at wavelengths of 560-590 nm (for example, see Non-patent document 4), a laser light source having an oscillation wavelength in this wavelength band is expected. Since lasers having oscillation wavelengths in this wavelength band are only large-sized lasers, such as a dye laser; a 532-nm solid-state laser and a 543-nm He—Ne laser are being used instead. However, since at these wavelengths, fluorescence wavelengths of green fluorescent proteins and absorption wavelengths of yellow fluorescent proteins overlap remarkably, these are inconvenient for measurement and analysis using multi-color fluorescent proteins.
Very recently, there is reported Kindling red fluorescent protein that emits red fluorescence stably for a long time of more than 72 hours by irradiation of intense green laser beam (wavelengths of 530-560 nm) (for example, see Non-patent document 5). An effect that the use of Kindling-Red fluorescent protein enables long-time observation of how a cell divides using fluorescence and other effects are expected. However, with the conventional 532-nm solid state laser and the 543-nm He—Ne laser, overlap between the fluorescence wavelengths of green fluorescent proteins and the absorption wavelengths of yellow fluorescent proteins is are significant. Therefore, realization of a compact solid-state laser having an oscillation wavelength as close to 560 nm as possible is desired.
Moreover, metalloporphyrin is a molecule contained in a protein that bears an important function for life activity of animals and plants, such as photosynthesis and respiratory metabolism, having an absorption maximum near the 590-nm wavelength. Since these emission wavelengths of metalloporphyrin exhibit peaks near 600 nm, if a laser of the 589-nm wavelength is used, overlap with the emission wavelengths is too large to perform measurement. Consequently, a golden yellow laser of the 585.0-nm wavelength is needed.
Furthermore, the 546.1-nm wavelength (yellowish green) corresponding to one of emission lines (e-line) emitted from a mercury vapor lamp is a wavelength at which human's visibility is highest, being used as a wavelength of the refractive index standard for optical glasses. As shown in FIG. 1, in the green, yellowish green, yellow ranges of 500-600 nm included in wavelength band 111, efficient and highly stable laser light sources are needed.
However, as described above, semiconductor lasers have been put to practical use only in the wavelength bands of shorter than 500 nm and longer than 620 nm. In the wavelength band of 500-620 nm, a solid-state laser of an arbitrary wavelength has not yet been realized. In order to generate light in the yellow range by the SHG method, a light source of a wavelength of 1092.2 nm, 1120.0 nm, or 1170.0 nm is needed. However, although the semiconductor lasers can oscillate at these wavelengths, it is very difficult to obtain a laser capable of delivering necessary output.
As described above, in making use of nonlinear optical phenomena, it is very important to select an existing laser that can deliver high excitation beam intensity and that is compact and consumes low power as well as improving the characteristics of a nonlinear optical crystal.
The second object of this invention is to provide a laser light source for generating a coherent beam in the yellow range that has a narrow linewidth and excellent directivity and is energy-efficient.
From the viewpoint of environmental protection as well as health and safety, it is strongly desired to establish ultralow volume analytical techniques of environmental gases, such as NOx, SOx, and ammonia system, absorption peaks of water, many organic gases, and residual pesticides. As the ultralow-volume analytical techniques, a quantitative analysis in which a gas to be measured (measured gas) is adsorbed in a specific substance and an electrochemical technique is performed, and an optical method for measuring optical absorption property intrinsic to a measured substance are common. Among these, the optical method has features that real-time measurement is possible and a widespread area through which measuring light passes can be observed.
Absorption peaks of a measured substance result form vibration modes of an interatomic bonding, and exist mainly in the mid-infrared region of 2-20 μm. However, in wavelength band 112 in the mid-infrared region shown in FIG. 1, a laser capable of continuous oscillation at room temperature has not yet been put to practical use, but only research and development of the quantum cascade laser is being advanced. Industrially, although the need for mid-infrared light is high, a fact that there is no practical laser light source becomes a large obstruction to applications.
Since there is no practicable light source in the mid-infrared region, when performing microanalysis of various gases etc. using existing semiconductor lasers (0.8-2 μm) for communications, absorption at the second overtone of the fundamental absorption wavelength (=½ of the fundamental absorption wavelength) and at the third overtone. (=⅓ of the fundamental absorption wavelength) will be used. As far as the second overtone is concerned, required sensitivity may be obtained. However, measurement at a high-order absorption peak of the third or higher overtone comes with a limit in detection, because the amount of absorption itself is small. Therefore, this method will bring decrease in sensitivity by about three orders of magnitude as compared to the measurement at the original fundamental absorption wavelength.
Therefore, in order to obtain high detection sensitivity in analyzing environmental gases, gases involving risk, etc., it is indispensable to develop a mid-infrared laser light source. In recent years, it was reported that mid-infrared light was generated in the vicinity of the 3-μm wavelength, and an operation as a gas sensor was verified (for example, see Non-patent document 6). A light source used in a gas sensor generates mid-infrared light by difference frequency generation using a lithium niobium oxide (LiNbO3) wavelength converter device that has a periodically poled structure.
However, the wavelength converter device having the periodically poled structure generates only mid-infrared light of a single fixed wavelength. Then, in order to make the wavelength variable so that different kinds of gases can be detected together, several methods are known as follows. (1) Several periods are provided in a single wavelength converter device (for example, see Non-patent document 7). (2) Period is changed by means of a structure called Fanout Grating (see the aforesaid Non-patent document 6). (3) Effective period is changed by making an excitation beam incident on the device slantingly (for example, see Non-patent document 8)
Although these methods can sweep a wavelength in a wide range, since the element with various periods had to be bundled, there was a problem that many operation processes were needed. Moreover, the technique of making an excitation beam incident on an element slantingly comes with a problem that it is difficult to create a waveguide structure in the device to attain high efficiency.
The third object of this invention is to provide a laser light source capable of tuning a laser beam in the mid-infrared region between 2-μm and 3-μm wavelength.
In recent years, environmental problem is coming to the fore greatly, and especially, attention centers on influences of dioxin on human body. In an incinerator that is one of origins of dioxin, generation of dioxin can be suppressed by controlling the combustion state of the furnace. For monitoring the combustion state, thermometers, CO concentration meters, and oximeters are needed.
As one technique to detect gas concentrations, there is known a method in which measured gases are irradiated with a laser beam and their absorption properties are observed. Since each gas has intrinsic absorption lines, the gas concentration can be detected by scanning a laser beam having a wavelength near the absorption line and observing an absorption spectrum. Points required for the laser beam at this occasion include monochromaticity, i.e., being a single-mode laser beam, delivering an output of a few mW to a few tens mW suited to gas detection, capability of stable wavelength scanning, long life, etc.
A laser beam used in the oximeter is in wavelength band 113 including a plurality of oxygen absorption lines existing at wavelengths of 759 nm to 768 nm, so gallium arsenide semiconductor lasers are being used (for example, see Patent document 1). A gallium arsenide semiconductor laser is manufactured by growing semiconductor crystals whose lattice constants almost agree with the lattice constant of gallium arsenide.
Semiconductor lasers are divided into the edge emitting type laser whose waveguide is manufactured in parallel to a substrate and the surface emission-type laser that emits light perpendicular to a substrate. Regarding gallium-arsenide edge emitting type lasers, relatively high-power single-mode lasers have been developed, but do not have structures to control their oscillation wavelengths. Consequently, the oscillation wavelength of the gallium-arsenide edge emitting type laser is determined at a point at which a gain peak of the active layer and a resonant mode of the resonator coincide. Therefore, the laser easily jumps among longitudinal modes at the time of wavelength scanning, and stable wavelength scanning is hard to perform.
As structures for controlling an oscillation wavelength, the distribution feedback (DFB) type, the distribution Bragg-reflection (DBR) type, etc. are well known. For these structures, it is necessary to manufacture semiconductor crystal whose refractive index is varied periodically in a direction parallel to the substrate, namely, whose composition is varied, in the semiconductor crystals. A manufacture method is that the surface of the semiconductor crystal is etched to a periodical structure, such as a corrugated shape, and thereon a semiconductor crystal of a different composition is grown. If the laser is intended to oscillate at the 763-nm wavelength in order to detect the oxygen concentration, it is necessary to suppress absorption at the wavelength and crystals of high aluminum concentrations must be used. However, if the aluminum concentration is high, there is a problem that the crystal is likely to be oxidized when manufacturing the periodic structure.
The surface emission-type laser is a kind of the DBR laser. In the surface emission-type laser, since a direction of emission is perpendicular to the substrate, the laser needs a DBR structure having a refractive index distribution in the perpendicular direction to the substrate. That is, it is only necessary to grow semiconductor crystals each of which is a layer parallel to the substrate and has a different composition so as to form a periodically stacked layers of crystals. Sine the manufacture can be completed with one round of semiconductor crystal growth, the manufacture is easy. However, since light passes through the active layer in a vertical direction in the surface emission-type laser, large gain cannot be obtained. In order to obtain sufficient output, a method for increasing the area of emission is conceivable. However, if the area of emission is increased, the laser will oscillate in a plurality of transverse modes, departing from a single-mode operation. If emission intensity of an order of mW necessary for detection of oxygen concentration is intended to be obtained while keeping a single-mode operation with a limited area of emission, current necessary for emission will concentrate in a minute area to increase the current density. For this reason, there is a problem that a life of the surface emission-type laser becomes as short as a few months.
The fourth object of this invention is to provide a laser light source that is high-power and long-life at wavelengths of 759 nm to 768 nm that are the oxygen absorption lines.
[Patent document 1] Japanese Patent Application Laid-open No. 6-194343(1994)
[Patent document2] U.S. Pat. No. 5,036,220
[Patent document 3] Japanese Patent Laid-open No. 4-507299(1992)
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